'Candidatus Microthrix parvicella' is a filamentous bacterium that grows with great difficulty in cultures from the mixed liquor of activated sludge sewage treatment plants. It is gram positive, and the ultrastructure of its cell walls has been determined to be of the gram-positive type by electron microscopical examination. Phylogenetically, it is a deep-branching member of the subphylum actinomycetes within the gram-positive phylum of the domain Bacteria. As for phenotypic features, it is known that the organism contains polyphosphate inclusions and that it is catalase positive. In mixed cultures in activated sludge plants and in pure culture in the laboratory, it has a characteristic and distinctive winding filamentous morphology, with filaments hundreds of micrometers long.